This invention relates to rotational rheometers and in particular to measuring cups for use in rotational rheometers.
Rotational rheometers (which term is intended to encompass inter alia rotary viscosimeters) are known in the prior art and typically comprise; a measuring cylinder driven to rotate by a motor via a shaft, the cylinder is received in a measuring cup which also receives a sample of material whose viscosity is to be measured. It is also known to rotate the measuring cup and sample relative to the measuring cylinder. Since temperature can affect the viscosity of many materials quite dramatically, the measuring cup, containing the sample, is usually maintained at a fixed temperature by a temperature control system.
Viscosity of a sample can be determined by measuring the torque exerted by the sample in resistance to the rotation of the measuring cylinder, parameters such as the current consumption of the motor and the phase shift of the electronic signal supplying the motor can be used to determine the torque.
Examples of rotational rheometers (more specifically rotary viscosimeters) known in the prior art are described in more detail in U.S. Pat. No. 6,240,770 B1. That patent discloses a novel combination of measuring cup and heat control system for maintaining a substantially uniform temperature distribution in the cup. The combination comprises a measuring cup surrounded by a heat control cup. An isolation gap is provided between the two cups to provide good heat isolation for the measuring cup. The two cups are in heat conducting contact in the vicinity of their upper circumferences to restrict heat conduction between them to that area. The measuring cup is constructed of a material with good heat conductivity, typically aluminium or copper. Heat is supplied to the heat control cup and is carried, by conduction, to the measuring cup where the two are in contact adjacent the rim.
It is an object of the present invention to provide an alternative means for ensuring that the temperature of a sample contained in a measuring cup of a rotational rheometer is maintained uniformly.
It is a further object of the invention to provide a rotational rheometer having a heat control system with improved effectiveness and which permits more improved accuracy in measurement of a sample""s viscosity.
In accordance with the invention, a rotational rheometer is provided with a heat control cup which has a wall thickness which decreases gradually from the bottom of the cup to its outer rim. A measuring cup can be slidably received within the heat control cup and is in heat conducting communication with the heat control cup. An air gap can, optionally, be provided between the two cups, provided there is some direct heat conducting contact between them, but it is preferred there is no air gap. Temperature of the heat control cup is maintained by a heat pump, such as a Peltier block. Heat is transferred to the measuring cup through the heat conductive communication between the measuring cup and the heat control cup. The heat control cup is constructed from a material with good heat conductivity such as aluminium, copper, alloys of aluminium or copper and the like. The temperature of the sample may be monitored by means of sensors which may, for example, be located in the wall of the heat control cup and/or the sample cup.
Desirably, the walls of the heat control cup decrease in thickness continually and uniformly from the bottom of the cup to the top. The decrease in thickness may be at a monatonic level. The heat control cup is desirably surrounded by a good heat insulator. Desirably the walls of the heat control cup comprise an inner surface which extends substantially perpendicularly from the base of the cup and an outer surface which is positioned at an angle to the inner surface which is an acute angle with respect to the base of the cup. The measuring cup desirably has a wall with parallel outer and inner surfaces which are substantially perpendicular to the base of the cup. The outer surface of the measuring cup is desirably of complementary shape and size to the inner surface of the heat control cup.